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From Andrew Wang <andrew.w...@cloudera.com>
Subject Re: Feature request to provide DFSInputStream subclassing mechanism
Date Wed, 07 Aug 2013 18:30:53 GMT
I don't think exposing DFSClient and DistributedFileSystem members is
necessary to achieve what you're trying to do. We've got wrapper
FileSystems like FilterFileSystem and ViewFileSystem which you might be
able to use for inspiration, and the HCFS wiki lists some third-party
FileSystems that might also be helpful too.

On Wed, Aug 7, 2013 at 11:11 AM, Joe Bounour <jbounour@ddn.com> wrote:

> Hello Jeff
> Is it something that could go under HCFS project?
> http://wiki.apache.org/hadoop/HCFS
> (I might be wrong?)
> Joe
> On 8/7/13 10:59 AM, "Jeff Dost" <jdost@ucsd.edu> wrote:
> >Hello,
> >
> >We work in a software development team at the UCSD CMS Tier2 Center.  We
> >would like to propose a mechanism to allow one to subclass the
> >DFSInputStream in a clean way from an external package.  First I'd like
> >to give some motivation on why and then will proceed with the details.
> >
> >We have a 3 Petabyte Hadoop cluster we maintain for the LHC experiment
> >at CERN.  There are other T2 centers worldwide that contain mirrors of
> >the same data we host.  We are working on an extension to Hadoop that,
> >on reading a file, if it is found that there are no available replicas
> >of a block, we use an external interface to retrieve this block of the
> >file from another data center.  The external interface is necessary
> >because not all T2 centers involved in CMS are running a Hadoop cluster
> >as their storage backend.
> >
> >In order to implement this functionality, we need to subclass the
> >DFSInputStream and override the read method, so we can catch
> >IOExceptions that occur on client reads at the block level.
> >
> >The basic steps required:
> >1. Invent a new URI scheme for the customized "FileSystem" in
> >core-site.xml:
> >   <property>
> >     <name>fs.foofs.impl</name>
> >     <value>my.package.FooFileSystem</value>
> >     <description>My Extended FileSystem for foofs: uris.</description>
> >   </property>
> >
> >2. Write new classes included in the external package that subclass the
> >following:
> >FooFileSystem subclasses DistributedFileSystem
> >FooFSClient subclasses DFSClient
> >FooFSInputStream subclasses DFSInputStream
> >
> >Now any client commands that explicitly use the foofs:// scheme in paths
> >to access the hadoop cluster can open files with a customized
> >InputStream that extends functionality of the default hadoop client
> >DFSInputStream.  In order to make this happen for our use case, we had
> >to change some access modifiers in the DistributedFileSystem, DFSClient,
> >and DFSInputStream classes provided by Hadoop.  In addition, we had to
> >comment out the check in the namenode code that only allows for URI
> >schemes of the form "hdfs://".
> >
> >Attached is a patch file we apply to hadoop.  Note that we derived this
> >patch by modding the Cloudera release hadoop-2.0.0-cdh4.1.1 which can be
> >found at:
> >http://archive.cloudera.com/cdh4/cdh/4/hadoop-2.0.0-cdh4.1.1.tar.gz
> >
> >We would greatly appreciate any advise on whether or not this approach
> >sounds reasonable, and if you would consider accepting these
> >modifications into the official Hadoop code base.
> >
> >Thank you,
> >Jeff, Alja & Matevz
> >UCSD Physics

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